1. School of Physical and Mathematical Sciences; Nanyang Technological University; 637371 Singapore
2. Natural Sciences and Science Education; National Institute of Education; Nanyang Technological University; 637616 Singapore
3. State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering; Zhejiang University; Hangzhou 310027 P. R. China
4. Institute of Materials Research and Engineering; A*STAR (Agency for Science, Technology and Research); #08-03, 2 Fusionopolis Way Innovis 138634 Singapore